Prior patent specification GB2090264 discloses a solution polymerisation process for the preparation of polymeric materials comprising cross-linked particles which are capable of forming sols, that is to say, a class of hydrogels which are referred to as microgels. The process involves polymerising one or more monomers in a solvent having particular characteristics and terminating the polymerisation before macrogelation occurs.
Microgels may be defined as intramolecularly cross-linked macromolecules. In common with other cross-linked polymers, microgels have a cross-linked structure and fixed surfaces. On the other hand, microgels may generally speaking be dissolved in certain solvents in the same way as non-cross-linked linear or branched polymers of similar molecular weight. In conventional cross-linked polymers, macrogelation occurs such that an extensive three-dimensional network is set up, which generally speaking resists dissolution in solvents. In microgels, the cross-linking structure exists predominantly within individual globular molecules.
The special molecular structure of microgels and their ability to exist as globular particles makes the microgel a promising material for pharmaceutical applications, such as carriers for controlled drug delivery. Patent specifications GB2090264, GB2143733 and GB2230952 disclose sustained release devices comprising an active ingredient and a hydrogel.
It is therefore foreseen that microgels may have a variety of potential industrial uses, and it would be desirable to provide an improved production process capable of producing microgels in a simple efficient and economical manner. Originally, microgels had to be produced at high dilutions which favour intramolecular rather than intermolecular cross-linking. Patent specification GB2090264 exemplifies an improved process for the production of microgels at higher concentrations in solvents having particular defined characteristics and under certain conditions which favour microgel formation. Generally speaking, the microgels formed have good solubility in the solvents used in the production process, so that solid microgel granules are generally obtained by precipitation of the microgel from solution by the addition of an organic liquid such as hexane, cyclohexane, petrol ether or methanol. However, the addition of a further organic liquid to the solvent used in the microgel production reaction means that the solvent cannot be directly reused without costly procedures for recovering the solvent from the mixture formed with the organic liquid. Furthermore, the microgel granule size or shape may not be suitable for direct use as a tabletting excipient, so that further processing such as grinding and sieving may be needed. These expedients are undesirable and detract from the industrial applicability of such microgel production processes.
It is an object of the present invention to mitigate these disadvantages.